Heart Disease

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Topics Addressed

  1. Experimental Design
  2. Medicine
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To measure the effect of exercise on heart disease, investigators in a study carried out in 1961 decided to compare the incidence of the disease for two large groups of people who worked on the London bus system: drivers and conductors. The conductors got a lot more exercise than the drivers as they walked around all day collecting fares while the drivers just sat there. The age distributions of the two groups were very similar, and all the subjects in the study had been on the same job for a period of at least ten years. The incidence of heart disease was substantially lower among the conductors.

Identify the treatment and outcome variables here, and explain briefly why this is an observational study rather than a controlled experiment. Should they have run a controlled experiment?
The investigators seem to have been worried that the age and length of service variables were potential confounding factors (PCFs); is this the case?
Has an association between exercise and heart disease been established by this study? What about causation? Are there any other confounds not yet mentioned that might explain these results?

Explain briefly.

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George Michailides
gmichail@stat.ucla.edu